The oil and gas drilling industry has been undergoing dramatic technology improvements in the last decade, particularly in MWD (Measurement-While-Drilling), directional and horizontal drilling, improved drilling tools and equipments, particularly the PDC bits, and improved analysis and monitoring capabilities. The combined effect is that drilling cost has been steadily declining, and directional drilling, particularly high-angle, extended reach, and horizontal drillings have become much more popular, and will have expanded application in the future.
At the same time, due to major operators' cost cutting efforts and down-sizing, more and more wells are being drilled on a "turn-key basis, whereby service companies are asked to contract the entire drilling project at a predetermined benchmark fee, with huge incentives for faster and better drilling, and similar penalties for incurring drilling troubles and drilling delays.
The advent of these turn-key projects creates the economic condition under which service companies that are able to improve aspects of the entire drilling operation will reap major profits, while those who do not can suffer major losses. One single severe incident of a stuck-pipe, or a lost drill string, or a side track can mean a loss of hundreds of thousands, if not millions, of dollars in revenue loss.
Furthermore, drilling transcends beyond the drilling phase. Proper drilling has medium and long term impacts on the total economy of the well in many areas: completion and work over; reservoir maintenance, as well as reservoir recovery economy. If the drilled well has many problems, including over-crookedness (called "dogleg"), overgaged hole due to the need for reaming, etc., there will be additional cost in drilling fluid, and more problems in running and setting casing, cementing, completion, as well as potential plugging of the well due to an oversized wellbore. Conversely, by drilling a longer and smoother horizontal section, one can dramatically improve recovery economy, making thin producing zones more economical to develop, improve completion operation, improve cementing, and reduce the chance of plugging. In other words, to have the most profitable producing well, it is necessary to drill the best well possible.
A key to improving the quality of the drilled well path is to make the well trajectory as smooth as possible. To achieve this goal, in addition to the many ways to control the inclination angle of he well while drilling that are presently available, it is necessary to have very good azimuthal control, particularly in the horizontal section. This will greatly reduce the chance of having drilling troubles, and minimize the need for course corrections.
When drilling a long reach horizontal well, where the horizontal section is generally in formations that are reasonably isotropic, it is very desirable to use "anti-walk" bits. This will allow true azimuth control with minimum course correction necessitated by azimuthal drift. Such drift phenomenon is very apparent in downhole motor drilling which has a very high side force at the bit, due to the bent housing induced interference fit. Current bits have unquantified and uncontrollable walk tendencies. The result is that periodic tool face adjustments must be made in order to Guide the drill in the proper direction. This creates wavy profiles in the well trajectory, which will cause Greatly increased torque and drag, leading to stuck pipe and other serious drilling and completion problems.
A key to preventing pipe sticking is to improve the quality of the drilled well path, namely, to make the well trajectory as smooth as possible. This will reduce the torque and drag generated by the friction due to contacts of the drillstring with the borehole wall.
In order to achieve these objectives, it is essential that we understand the inherent deviation tendencies of the bit and BHA (bottomholeassembly) system, which may include a downhole motor with or without a bent housing.
There are no existing theories or models describing the inherent walk tendency of PDC bits. Attempts in quantifying the overall deviation tendencies of either the bit or the BHA started with a basic mechanics study of the BHA deformation analysis by Lubinski et al. "Factors Affecting the Angle of Inclination and Doglegging in Rotary Bore Holes", API Drilling & Prod. Pract., 1953, which included the formation anisotropy effects. Many works have been made since then on the study of either the effect of BHA deformation on the build-drop tendencies of the entire assembly, or the effect of bit's anisotropic drilling characteristics on the drilling deviation, or both.
The most comprehensive theory and model to date is that by the present inventor in "Prediction of Drilling Trajectory in Directional Wells Via a New Rock-Bit Interaction Model", SPE Ann. Conf., Paper #16658, 1987 and in "General Formulation of Drillstring Under Large Deformation and its Use in BHA Analysis", SPE Annual Technical Conf. and Exh., October 1986. These articles accounted for both the effects of anisotropy in both the bit and the rock, in addition to the side forces generated by the BHA, which is to be computed by a BHA analysis program which includes, for the first time, the nonlinear effects of drillstring's "large" deformation, particularly for medium radius drilling where the borehole's well path has a curvature (build or drop rate) of 10' to 30' per 1000' of well path.
This model was further described in U.S. Pat. No. 4,804,051, issued on Feb. 14, 1989 to the present inventor. This patent describes the model which accounts for anistropic drilling characteristics of both the formation and the bit. It is capable of predicting the walk tendency and the build-drop tendency of a given BHA (bottomhole assembly) under any drilling condition. The model can be used in the forward mode to predict the drilling direction; in the inverse mode to generate the rock and bit anisotropy indices; and in the log-generation mode to generate drilling logs, such as a drilling dip log.
However, even in the referenced model by Ho, the way the anisotropy effects are accounted for is through a "global" correlation, using the rock and bit anisotropy indices. It has the following limitations: (1) Irrespective of any detailed PDC bit cutter arrangements, a single bit anisotropy index is sufficient to describe the deviation tendency of the bit; (2) When drilling in isotropic formations, the bit axis, the total bit force, and the drilling direction are coplanar such that the PDC bits will not walk; (3) The rock anisotropy index is assumed to be an inherent formation property, and independent of the bit's cutter design, so as to be independent of the bit anisotropy index, or the detailed cutting action of the bit; and (4) The model does not work when drilling through formation interfaces. This may be a significant drawback, particularly when drilling through alternating hard and soft sand-shale sequences.
Field experiences have shown that PDC bits generally walk left, although some bits walk right. However, such field observations do not separate the effects of formation anisotropy, formation dip, the BHA design, as well as the effect of borehole trajectory and inclination. Although some nonlinear effects can be generated in drilling in a curved well path, the effect is small, and cannot account for all the walk tendencies that have been observed. Formation anisotropy can also induce walk tendency.
Failure to account for the walk tendency of the PDC bit--BHA system will result in frequent course corrections. Such course corrections may require tripping in rotary drilling, which is very time consuming and therefore very expensive. In downhole motor drilling, such course corrections are achieved through frequent tool face adjustments. Though this eliminates the tripping time, it increases the well path's crookedness, which greatly increases the chance of the assembly getting stuck, resulting in major expenses. Furthermore, the fear of getting stuck particularly in the horizontal section of the well, places a major limitation on the length of the horizontal reach one can drill into. This limitation inhibits the major economic potential of drilling such horizontal wells.
One technique employed in the past has been the development of bits having a prescribed walk tendency. U.S. Pat. No. 5,099,929, issued on Mar. 31, 1992, to Keith et al. describes an unbalanced PDC drill bit having right hand walk tendencies. A plurality of stationary cutter elements are fixedly mounted to the cutting face in a selected pattern so as to provide a region of high cutter density on one side of the cutting face, and a region of low cutter density on the other side of the cutting face. The stationary cutter elements operate to cut into the lower side wall of the wellbore as the bit body is rotated, causing the rotary drill bit to walk to the right.
It is an object of the present invention to provide a method that allows the deviation characteristics of the PDC bit to be determined.
It is another object of the present invention to provide a method whereby PDC bits can be designed with known quantitative walk tendencies.
It is a further object of the present invention to provide a method whereby the walking tendencies of PDC bits can be determined so as to allow for the planning of well paths in a manner most suitable for the bits.
It is a further object of the present invention to provide a method that can enable the completion of a long bit run without trippings or course corrections.
It is a further object of the present invention to provide a method whereby formation dip can be determined from walk tendencies of the bit.
It is a still another object of the present invention to provide a method which greatly reduces the total drilling cost of directional wells, and which provides greater ease in completion and cementing.
It is a further object of the present invention to provide a method which serves to reduce the total developmental cost of the oil and gas field and to enable the development of currently marginal fields.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.